There are many industrial and environmental processes and industries that require the storage or holding of fluids in various forms of containers or tanks for varying periods of time, from minutes to months or years. Typical examples would include oil storage tanks, sewage holding and treatment tanks, and processing and storage tanks as used, for example, in paper manufacturing and other chemical processes.
A common requirement in such holding and storage tanks is that of sensing or measuring the level of fluid in the tanks, for example, to warn when a tank is full or should be filled, to control the pumping of fluid into or from a tank so as to avoid overflow or pump damage when a tank is empty, and to otherwise control or measure the level of fluid in a tank. A recurring problem with sensing or measuring the level of fluid in a tank or other form of container, however, is that many of the fluids contain or are comprised of substances that leave or form deposits on the inner surfaces of the tanks that interfere or prevent the measurement or detection of the fluid levels. Such deposits, referred to hereafter as "sludge" , may be comprised of solids dissolved or suspended in the fluids or components of the fluids themselves and typical examples would include the solid and semi-solid or semi-liquid components of sewage, components of petroleum products, the fibrous components of paper "slurry", and chemical and mineral deposits, such as "scale".
The effects of sludge buildup on fluid level sensing devices that were mounted internally to a tank were recognized and, although still used at the cost of frequent repair and cleaning, often under hazardous conditions, are still used. More recent developments, however, have been directed at externally mounted sensors that sense or measure some property of a tank that changes dependent upon the level of fluid therein. A typical example of such is described in U.S. Pat. Ser. No. 5,456,114 to Liu et al. for an ELASTIC WAVE SENSING SYSTEM, hereafter referred to as "Liu", which uses a transmitting transducer and a receiving transducer mounted on the outside of a tank wall to propagate a relatively high frequency elastic wave through the wall of the tank. The speed of propagation of the elastic wave through the wall of the tank is affected by whether there is fluid present against the inner wall of the tank along the propagation path and the change in time of propagation of the wave between the transmitting and receiving transducers is measured by a zero crossing technique to determine whether fluid is in contact with the inner wall of the tank along the propagation path. One implementation of this approach has been as a yes/no level detector using two closely spaced transducers, optionally with an associated "calibration" path comprised of a small container of water positioned as a parallel path between the transducers, to sense the presence or absence of fluid in the tank at the level of the transducers. Another implementation has placed one transducer at the bottom of the tank wall the other at the top of the tank wall and measured the change in propagation time between the transducers as an indicator of the proportion of the path between the transducers having fluid against the inner wall of the tank.
This method as taught by Liu uses an elastic wave signal in the frequency range of 5 to 25 kHz, typically at 12.5 kHz, and has been found to perform satisfactorily in situations where the inner surfaces of the tank are relatively clean of sludge or other deposits, such as large storage tanks for "clean" oil, such as heating or diesel oil. It has been found, however, that this method is not satisfactory in situations wherein a layer of "sludge" forms or is deposited on the inner wall of a tank or container along the propagation path of the elastic wave. For example, many ships are provided with sewage storage tanks for holding waste, thereby avoiding the necessity of frequent or continuous dumping of sewage into the surrounding waters and allowing the sewage to be disposed of in more environmentally acceptable ways. It has been found, however, that the inner walls of such tanks are typically covered with one to two inch thick layers of sludge comprised of solid, semi-solid and semi-liquid substances, usually saturated with water, and having density on the same order as that of the fluid held in the tank. It has also been found that a layer of such sludge having a thickness of even one eighth of an inch will appear to a fluid sensing detection system as embodied by Liu et al. to be the fluid that the system is trying to detect, thereby resulting in false or erroneous readings or indications of fluid levels.
The present invention provides a solution to these and other problems of the prior art.